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Spring 2015

When Confinement Gives You a Speed Boost

Nanopores help squeeze out the competition to improve the power of supercapacitors

Robert L. Sacci

The molecular-scale properties of a carbon electrode for supercapacitors with meso- and micro-pores was examined using neutron scattering probes and molecular simulations. Reprinted with permission from Bañuelos et al. Copyright 2014 American Chemical Society.

Molecules and ions behave strangely when they get into tight places. The researchers at the Fluid Interface Reactions, Structures and Transport (FIRST) Center are studying how molecules and ions move through tight spaces and how to design "highways" that increase the speed of ions. They found that nano-sized tubes, 1,000 times smaller than a strand of hair, act as molecular highways that can increase the power of energy storage devices.

Allowing ions to travel faster is important for high power density devices, such as supercapacitors. These devices allow for rapid storage and use of electric power. They do this by storing electric charge at the solid-liquid interface of high-surface-area porous carbon electrodes.

Porous carbon electrodes have a collection of pores with widths from microns down to sub-nanometers, smaller than the width of a single DNA strand. Previous efforts in the FIRST Center showed that decreasing pore width increases the amount of energy that the pore can store. However, as pores become nano-sized, decreasing the pore size produces erratic results; the stored energy can either increase or decrease. This effect lasts until the pore becomes 10,000 times smaller than a strand of hair. That is, the charge storage capacity drastically increases when the pores act as nano-cages just a little bigger than the "trapped" ions.

Leo Bañuelos and co-workers at the FIRST Center have studied the dynamics of room-temperature ionic liquids within the carbon electrode's nanometer-sized pores and how those dynamics can affect the storage of electric charge. Few techniques allow probing the average structure and motion of molecules and ions at the nanometer and sub-nanometer scale within samples containing pores of different sizes. With this in mind, Banuelos and co-workers turned to the Spallation Neutron Source at Oak Ridge National Laboratory to probe the structure and dynamics of the ionic liquids within the meso- and micro-pores.

Mesopores are pores between 2 and 50 nanometers—a few hundred ions in diameter. Micropores, however, are less than 2 nanometers. In other words, ions within micropores are confined in nano-sized cages.

"Small angle neutron scattering allows us to see inside optically impenetrable nanometer-size porous materials and find out how ionic liquid ions are reorganized under confinement," explained Banuelos. "Then, using neutron spin echo spectroscopy, we were able to measure ion dynamics throughout different regions of the hierarchical porous carbon." The researchers also used computational modeling to simulate how ionic liquids are arranged near the pore walls and how the ions move throughout the pores.

They found that the carbon surface disrupted the ionic liquid bulk structure within the slit-like micropores, while the larger mesopores acted as molecular highways allowing ions to quickly enter the micropores. Once contained within the micropores, the speed of the ions decreased; however, the nano-cages allowed the ions to store more energy.

This study showed how neutron techniques at facilities such as the Spallation Neutron Source and the Center for Neutron Research at the National Institute of Standards and Technology can be used to track molecular motion within carbon electrodes and drive the design of improved energy storage materials.

Banuelos went on, "The best electrodes for supercapacitors must have large 'highways' [mesopores] for fast ion transport to the high energy storage sites [nano-cages]. These results help unravel the structure and dynamics, across several length and time scales, of an electrolyte inside a real electrode material."

More Information

Bañuelos JL, G Feng, PF Fulvio, S Li, G Rother, S Dai, PT Cummings, and DJ Wesolowski. 2014a. "Densification of Ionic Liquid Molecules within a Hierarchical Nanoporous Carbon Structure Revealed by Small-Angle Scattering and Molecular Dynamics Simulation." Chemistry of Materials 26(2):1144-1153. DOI: 10.1021/cm4035159

Bañuelos JL, G Feng, PF Fulvio, S Li, G Rother, N Arend, A Faraone, S Dai, PT Cummings, and DJ Wesolowski. 2014b. "The Influence of a Hierarchical Porous Carbon Network on the Coherent Dynamics of a Nanoconfined Room Temperature Ionic Liquid: A Neutron Spin Echo and Atomistic Simulation Investigation." Carbon 78:415-427. DOI: 10.1016/j.carbon.2014.07.020

Acknowledgments

Bañuelos et al., 2014a: This work was supported by the Fluid Interface Reactions, Structures and Transport (FIRST) Center, an Energy Frontier Research Center funded by the U.S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences. The small-angle X-ray scattering portion of this research was conducted at the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, Office of Basic Energy Sciences, DOE. The small-angle neutron scattering portion of this research at Oak Ridge National Laboratory’s Spallation Neutron Source was sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, DOE.

Bañuelos et al., 2014b: This work was supported by the FIRST Center, an Energy Frontier Research Center funded by the DOE, Office of Science, Office of Basic Energy Sciences. The Spallation Neutron Source neutron spin echo portion of this research conducted at Oak Ridge National Laboratory’s Spallation Neutron Source was sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, DOE. The National Institute of Standards and Technology neutron spin echo portion of this work used facilities supported in part by the National Science Foundation.

About the author(s):

Squeezing Molecules for Energy Storage

Designing tight-fitting pockets on the electrode's surface increases its ability to store electricity

Allowing ions to travel faster is important for high power density devices, such as supercapacitors. Scientists are studying how molecules and ions move through tight spaces and how to design "highways" that increase the speed of ions.

Supercapacitors outlast today's batteries and handle more power. Yet, they don't pack the punch to run mobile phones for weeks between charges. To build energy-dense supercapacitors, scientists at the Fluid Interface Reactions, Structures and Transport (FIRST) Energy Frontier Research Center examined the behavior of an innovative electrolyte. Conventional theories do not explain how electrolytes behave on an electrode's surface when the electrolyte is composed entirely of ions. The team showed some surprising results. When the surface pores, or pockets, are less than a nanometer wide, a bit smaller across than a single strand of DNA, the molecules pack tightly and the electrode holds more energy. When the pores are slightly larger, the energy density doesn't increase, but the speed of the molecules does increase. Oak Ridge National Laboratory leads the center.

More Information

Bañuelos JL, G Feng, PF Fulvio, S Li, G Rother, S Dai, PT Cummings, and DJ Wesolowski. 2014a. "Densification of Ionic Liquid Molecules within a Hierarchical Nanoporous Carbon Structure Revealed by Small-Angle Scattering and Molecular Dynamics Simulation." Chemistry of Materials 26(2):1144-1153. DOI: 10.1021/cm4035159

Bañuelos JL, G Feng, PF Fulvio, S Li, G Rother, N Arend, A Faraone, S Dai, PT Cummings, and DJ Wesolowski. 2014b. "The Influence of a Hierarchical Porous Carbon Network on the Coherent Dynamics of a Nanoconfined Room Temperature Ionic Liquid: A Neutron Spin Echo and Atomistic Simulation Investigation." Carbon 78:415-427. DOI: 10.1016/j.carbon.2014.07.020

Disclaimer: The opinions in this newsletter are those of the individual authors and do not represent the views or position of the Department of Energy.